A thread forming tap is known that is provided with an external thread having a full thread portion and a leading portion disposed contiguous with the full thread portion and tapered toward the tip, both the full thread portion and the leading portion having lobes and recesses that are alternately formed thereon (see Patent Literatures 1 and 2). In such a thread forming tap, its leading portion is first screwed into a prepared hole disposed in a workpiece so that the lobes cut into inner wall surfaces of the prepared hole to cause a plastic deformation for the formation of an internal thread, with the result that no chips are created simplifying clearing works, etc.